1. Field of the Invention
This invention relates to improvements in a cylinder direct injection spark-ignition engine in which fuel (such as gasoline) is directly injected into a cylinder, and more particularly to the improvements for achieving effective stratified charge combustion and effective homogeneous charge combustion in such an engine.
2. Description of the Prior Art
Cylinder direct injection spark-ignition engines for an automotive vehicle have been known, in which fuel is directly injected into an engine cylinder to be ignited by a spark plug. One of them is disclosed in U.S. Pat. No. 5,553,588 entitled "Spark-ignited Direct Cylinder Fuel Injection Engine" and configured as follows: The engine has two intake ports for each cylinder, in which one of the intake ports is provided with a swirl control valve. Each piston is formed at its piston crown with a cavity forming part of a combustion chamber defined between the piston and a cylinder head. Fuel is injected from a fuel injector valve toward the cavity in the compression stroke thereby locally forming a layer of combustible air-fuel mixture around the spark plug, thus achieving stratified charge combustion. Additionally, under a certain engine operating condition, fuel is injected in the intake stroke to disperse fuel inside the combustion chamber thereby forming a homogeneous air-fuel mixture, thus accomplishing homogeneous charge combustion.
In order to accomplish effective stratified charge combustion in the cylinder direct injection spark-ignition engine, it is necessary to form stratified charge or air-fuel mixture inside the cylinder. In this regard, with the above conventional engine, atomized fuel injected from the fuel injector valve in the compression stroke is vaporized inside the cavity at the piston crown and carried to the vicinity of the spark plug under the action of gas flow (in the combustion chamber) regulated by the swirl control valve and under the action of a spray penetration force as a characteristic of the fuel injector valve. Thus, it is made possible to ignite a stratified air-fuel mixture, thereby accomplishing lean combustion.
However, the gas flow inside the cylinder is based on suction and compression of intake air under movement of the piston, and therefore the speed of the gas flow increases with an increase in engine speed or an increase in piston speed. Consequently, in the case of stratified charge combustion, it is required to keep air-fuel mixture around the spark plug in a good condition without being dispersed during a time period between a fuel injection timing and a spark ignition timing because the distance between the fuel injector valve and the spark plug is fixed.
In order to securely carry atomized fuel to the vicinity of the spark plug under strong gas flow, a sufficient fuel spray penetration force is required. This requirement can be met by concentrating the spray of atomized fuel, for example, by narrowing a spray angle of the fuel spray. However, concentration of the fuel spray tends to form a rich air-fuel mixture mass in the combustion chamber, so that soot is produced in the combustion chamber causing the spark plug to become sooty. In this regard, the magnitude of the gas flow in the combustion chamber increases in proportion to engine speed. Transportation control of stratified air-fuel mixture depends on the respective parameters of the gas flow and the fuel spray penetration force, in which a transportation system becomes stable by being controlled depending on the parameter of the fuel spray penetration force. However, in order to enhance a horizontal velocity component (directed to the spark plug), it is required to minimize the spray angle so as to increase the degree of concentration of the fuel spray. This invites problems such as production of soot in the combustion chamber.
Further, in order to accomplish effective homogeneous combustion in the cylinder direct injection spark-ignition engine, it is necessary to effectively vaporize atomized fuel and mix the atomized fuel with air preventing adhesion of the fuel to the wall of the combustion chamber. In view of this, it is proposed to make fuel injection at the initial stage of intake stroke to ensure a time for vaporizing fuel, which tends to improve the condition of air-fuel mixture in the combustion chamber and to increase power output of the engine. However, in case that the atomized fuel contacts the piston and the wall of the combustion chamber in a high concentration degree state of the fuel spray before dispersion of the fuel spray when the piston is near the top dead center, the atomized fuel adheres to the surface of the piston and of the combustion chamber wall thereby producing soot while inviting exhaust gas temperature rise and power output degradation due to insufficient mixing of fuel with air.
Otherwise, it has been proposed to form a deep cavity at the piston crown to increase the relative distance between the piston crown surface and a fuel injection position at the same fuel injection timing for the purpose of preventing interference of fuel spray with the piston. However, this increases the surface area of the combustion chamber so as to increase heat loss of the engine while impeding gas flow inside the cylinder, thus lowering power output of the engine.